A parallel optical communications module is a module having multiple transmit (TX) channels, multiple receive (RX) channels, or both. Parallel optical communications modules that have both transmit and receive channels are known as parallel optical transceiver modules. In parallel optical transceiver modules, the TX portion comprises components for transmitting data in the form of modulated optical signals over multiple optical waveguides, which are typically optical fibers. The TX portion includes a plurality of laser driver circuits and a plurality of laser diodes. The laser driver circuits output electrical signals to the laser diodes to modulate them. When the laser diodes are modulated, they output optical signals that have power levels corresponding to logic 1s and logic 0s. An optics system of the module focuses the optical signals produced by the laser diodes into the ends of respective transmit optical fibers held within a connector that mates with the module.
Typically, the TX portion also includes a plurality of monitor photodiodes that monitor the output power levels of the respective laser diodes and produce respective electrical feedback signals that are fed back to a module controller. The controller processes the feedback signal to obtain respective average output power levels for the respective laser diodes. The controller outputs control signals to the laser driver circuit that cause it to adjust the modulation and/or bias current signals output to the respective laser diodes such that the average output power levels of the laser diodes are maintained at relatively constant levels.
The RX portion of a parallel optical transceiver module includes a plurality of receive photodiodes that receive incoming optical signals output from the ends of respective receive optical fibers held in the connector. The optics system of the module focuses the light that is output from the ends of the receive optical fibers onto the respective receive photodiodes. The receive photodiodes convert the incoming optical signals into electrical analog signals. An electrical detection circuit, such as a transimpedance amplifier (TIA), receives the electrical signals produced by the receive photodiodes and outputs corresponding amplified electrical signals, which are processed in the RX portion to recover the data.
There is a constant demand in the optical communications industry for parallel optical transceiver modules that are capable of transmitting and/or receiving ever-increasing amounts of data at ever-increasing speeds. To accomplish this, it is known to combine multiple parallel optical transceiver modules of the type described above to produce a parallel optical communications system that has a higher bandwidth than the individual parallel optical transceiver modules. A variety of parallel optical transceiver modules are used in such systems for this purpose. For example, one known parallel optical transceiver module of the type described above includes a multi-fiber connector module known in the industry as the MTP connector module. The MTP connector module plugs into a receptacle that is secured to a front panel of a rack of the optical communications system. The MTP connector module receives a duplex fiber ribbon cable having a total of 4, 8, 12, 24, or 48 optical fibers. Typically, half of the fibers of the ribbon cable are transmit fibers and the other half are receive fibers, although all of the fibers may be either transmit or receive fibers in cases where the module is being used as either a transmitter or a receiver, but not both.
When the MTP connector module is plugged into the receptacle, electrical contacts of the connector module are electrically connected with electrical contacts of a printed circuit board (PCB) of the transceiver module. The laser diodes and the photodiodes are integrated circuits (ICs) that are mounted on the PCB. A laser driver IC and a transceiver controller IC are typically also mounted on the PCB, although the transceiver controller IC is sometimes mounted on a motherboard PCB of the optical communications system.
It is known that multiple transceiver modules of the type that use the MTP connector module can be arranged in an array to provide an optical communications system that has an overall bandwidth that is generally equal to the sum of the bandwidths of the individual transceiver modules. One of the problems associated with such an array arises from the fact that the MTP connector modules are mounted by plugging them into receptacles formed in a front panel of a rack of the optical communications system. Because the modules are mounted in this manner, there must be sufficient space on the front panel to accommodate the receptacles and the respective MTP connector modules. Because space on the front panel is limited, the ability to increase bandwidth by increasing the size of the array is also limited.
An alternative to the mounting arrangement described above is to mid-plane mount the parallel optical transceiver modules. A mid-plane mounting configuration is one in which the modules are mounted in the plane of the motherboard PCB. One known parallel optical transceiver module that is mid-plane mounted is known in the industry as the Snap 12 transceiver module. The Snap 12 transceiver module comprises a 12-channel TX module and a 12-channel RX module. Each module has an array of 100 input/output (I/O) pins that plugs into a 100-pin ball grid array (BGA). The BGA is, in turn, secured to a motherboard PCB.
Other mid-plane mounting solutions exist for mounting multiple parallel optical transceiver modules on a motherboard PCB. One of the problems associated with the existing mid-plane mounting solutions is that there are limitations on the mounting density of the modules on the motherboard PCB. Each module has its own PCB, ball grid array, or other type of internal mounting structure that is parallel to the motherboard PCB. Thus, each module consumes spatial area, i.e., has a footprint, on the surface of the motherboard PCB. In addition, each of the modules must be spaced apart from adjacent modules on the motherboard PCB by some minimum spacing, or pitch. Because there is a finite spatial surface area on the motherboard PCB for mounting the modules, the mounting density of the modules is limited, which limits the overall bandwidth of the system.
A need exists for an optical communications system having a mounting configuration that enables parallel optical communications modules to be mounted with increased mounting density. Increasing the mounting density of the modules increases the amount of data that can simultaneously be transmitted and/or received by the optical communications system.